In the age of emails and tweets, writing old-fashioned letters has become the new way of expressing dissent in Iran. The veteran journalist Mohammad Nourizad wrote his first open letter to Iran's supreme leader, Ayatollah Ali Khamenei, in the aftermath of the disputed 2009 election. It went unanswered, as have Nourizad's 21 other missives, but dozens more prominent Iranians have followed his example in a campaign to break the taboo on criticising the man who purports to be God's representative on earth.

Nourizad, an influential columnist and film-maker, was originally an ally of Khamenei who wrote for the ultra-conservative state-run newspaper Keyhan, whose director is directly appointed by the supreme leader.

But that changed amid the bloody crackdown on opposition protesters after the election. In his first letter Nourizad called Khamenei "father" – but criticised him for his handling of the unrest and called on him to apologise to his people.

"As commander in chief of the armed forces, you didn't treat people well after the election. Your agents opened fire, killed the people, beat them and destroyed and burnt their property. Your role in this can't be ignored ... Your apology can cool down the wrath of the people," he wrote.

The letter – and those which followed – infuriated the authorities, and although Nourizad, now 60, was for a while protected by his prominence, in April 2010 he was arrested on charges of insulting the officials and propaganda against the regime.

In jail, he spent almost 70 days in solitary confinement and was subject to lengthy interrogations without the presence of his lawyer, during which he says he was physically abused. Nourizad staged a hunger strike in protest, but was sentenced to three and a half years. He was released after 170 days.

Once outside prison, Nourizad continued to voice his criticism in open letters and later asked other activists to join his campaign.

The letters have offered a form of samizdat criticism, copied and shared among regime insiders. The former general whose letter Nourizad recently published said he was dismissed from the Revolutionary Guards for duplicating and distributing the journalist's letters.

Nourizad received death threats for his letter campaign. "I'm not afraid of being arrested, nor dying for what I am doing now," he told the Guardian in a phone call from Tehran. "In fact, I've put my life in the palm of my hand. The authorities can't do much with a person like me who is not even afraid of dying."

Nourizad is not the only one resorting to letter-writing to air his grievances. Many imprisoned activists followed suit, describing their time in custody to their family members in letters smuggled out of prison. Nasrin Sotoudeh, a human rights lawyer who has been jailed for six years, recently wrote an emotional letter to his young son on a piece of toilet paper.